1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device comprising field-effect transistors (FETs) which are used in memory cells and have a gate insulation film made of ferroelectric material.
2. Description of the Related Art
Known as a memory element is such a gate-insulated transistor as is shown in FIG. 1, which has a floating gate electrode 74. This memory element is used in an EPROM (Electrically Programmable Read-Only Memory).
To write data into the floating-gate transistor shown in FIG. 1, a high voltage is applied to the drain region 71 and the control gate electrode 72, causing an avalanche breakdown and injecting electrons into the floating gate through the gate insulation film 73, thereby to change the threshold voltage of the transistor. The voltage applied to the control gate electrode 72 is controlled to render the path between the drain region 71 and the source region 75, either conductive or non-conductive. However, whether this source-drain path is conductive or not depends on the threshold voltage of the transistor, which varies in accordance with the electrical charge of the floating gate electrode 74. Since the floating-gate transistor shown in FIG. 1 is an N-type one, its threshold voltage is low when no electrons have been injected into the floating gate electrode 74, and is high when electrons have been injected into the electrode 74.
The data, i.e., a "1" bit or a "0" bit, written in the transistor, can be detected in accordance with whether or not the source-drain path is conductive, or whether or not electrons are accumulated in the floating gate electrode 74 when a predetermined voltage is applied to the control gate electrode 72. To erase the data from the floating-gate transistor, it suffices to apply ultraviolet rays onto the transistor, thereby releasing the electrons from the floating gate electrode 74.
However, it takes much longer to write data into the transistor, than to read the data therefrom. This is because the data is written into the floating-gate transistor by applying a high voltage to the transistor, thus causing an avalanche breakdown and injecting electrons into the floating gate through the gate insulation film 73. Moreover, to erase the data from the floating-gate transistor, ultraviolet rays must be applied to the transistor, which requires a special device to apply the rays and also some time to do so.
In view of the drawbacks pointed out in the preceding paragraph, the floating-gate transistor is not suitable for use as a memory cell of a RAM (Random-Access-Memory) which write data within a time not greatly different from the time required to read the data.
Recently, a capacitor, which has an insulation film made of ferroelectric material and interposed between the two electrodes and which can function as a memory cell, has been disclosed in Electronics, Feb. 4, 1988, p. 32, and also in Electronics, Feb. 18, 1988, pp. 91-95). The polarization of the ferroelectric material is utilized to indicate the data. It is easy to write data into, and to read data from, this capacitor. Since the capacitor keeps storing the data even after the power supply to it is stopped, it is useful as a memory cell of a nonvolatile memory.
To write data into, or read data from, a capacitor which has a ferroelectric insulation film and is used as a memory cell of a nonvolatile RAM, the polarization of the insulation film must be inverted. Here arises a problem. Since the ferroelectric insulation film fails to function normally after it has undergone inversion of polarization a predetermined number of times, the capacitor cannot be used long as a memory cell.
On the other hand, the ferroelectric gate insulation film of a FET used as a memory cell need not undergo inversion of polarization to read data, though it must undergo this phenomenon to write data. Hence, the FET can be used longer as a memory cell than the capacitor described above. It is demanded that some measures be taken which make it possible to use FETs as memory cells practically.